The present invention relates generally to the field of digital signal processing circuitry, and more particularly to the field of digital electronic dwell circuits used in ignition control systems which control spark and dwell occurrence.
In internal combustion engines the time occurrence at which a spark is produced to ignite a fuel and air mixture in a cylinder is a primary operational consideration. Similarly, producing an appropriate excitation signal (dwell) for an ignition coil immediately prior to the coil producing spark ignition is also a major design consideration. Mechanical spark control ignition systems have been found not to be reliable over long periods of time thus necessitating frequent readjustment of the mechanical controls. Thus electronic dwell and spark control ignition systems having greater reliability have been developed.
Electronic dwell circuits for ignition control systems are known and U.S. Pat. Nos. 3,908,616 and 4,018,202 illustrate digital circuits for determining a dwell control signal. While the circuits shown in these patents evidently produce accurate digital dwell control signals, generally they are not economically adaptable to operate in conjunction with digital spark timing circuits in which the spark timing is to be a function of engine speed and other additional engine variables. This is because generally prior art dwell circuits utilize circuitry which is separate from the spark timing calculation circuitry. Thus the dwell calculation is accomplished without the utilization of the majority of the spark calculation circuitry, thus increasing the cost of the total ignition control system.
Generally, prior art dwell circuits such as U.S. Pat. No. 4,018,202 utilize a complex and costly cam structure having an extremely large number of individual teeth projections in order to produce a series of high resolution crankshaft position pulses, typically one pulse being produced for every one degree of crankshaft rotation. The construction of these cams is costly and their utilization would tend to inhibit utilization of the same cam to produce other crankshaft position pulses which would occur at other than one degree increments of crankshaft rotation. Of course this deficiency can be overcome by utilizing additional cams and additional crankshaft position sensors, but then the cost of the ignition control system would be increased. While the one degree pulses can be electronically realized by dividing up large angular crankshaft pulses, this would also add to the cost of an ignition control system.
The one degree crankshaft position pulses produced by the prior art dwell circuits represent speed dependent crankshaft position pulses and enable the prior art circuits to readily calculate ignition dwell as a fixed number of degrees of crankshaft rotation. However these circuits have problems in realizing a constant dwell time, rather than constant dwell angle, which is desired from some engine operative conditions. Also prior dwell circuits such as U.S. Pat. No. 4,018,202 require complex feedback circuits having marginal stability.
Some dwell circuits such as those in U.S. Pat. No. 3,908,616 utilize speed independent pulses in order to calculate ignition dwell. While these circuits have eliminated the need for a multi-tooth crankshaft cam or its electronic equivalent for producing high resolution crankshaft position pulses, the disclosed circuit designs cannot produce large dwell angles which are required at high engine speeds. In addition, the dwell circuit in U.S. Pat. No. 3,908,616 contemplates adjusting count thresholds in order to adjust the dwell occurrence and/or contemplates adjusting the rate at which pulse counting takes place. In order to implement either of these two functions, relatively complex and costly control structures are required.
Typically, digital signal processing circuits which intend to implement the function of producing a pulse occurrence a predetermined time prior to the known occurrence of periodic signal pulse transitions having a variable occurrence rate have utilized circuit configurations corresponding to those shown in U.S. Pat. Nos. 3,908,616 or 4,018,202. Therefore they have suffered from the same deficiencies described above.